Magnetic resonance imaging (MRI) systems typically include a superconducting magnet for generating a static magnetic field B0, and one or more special-purpose radio-frequency (RF) coils for generating a time-varying magnetic field B1 perpendicular to the field B0, and for detecting the response of a sample of interest (e.g. part of a human or animal) to the applied magnetic fields. The direction of the static magnetic field B0 is commonly denoted as the z-axis or longitudinal direction, while the plane perpendicular to the z-axis is commonly termed the x-y or transverse direction.
Several types of RF coils have been used in MRI systems, including solenoid coils, saddle-shaped coils, TEM resonators, and birdcage coils. Birdcage coils typically include two transverse rings, and a relatively large number of vertical rungs connecting the rings. Birdcage coils are multiply-resonant structures in which specified phase-relationships are established for current flowing along multiple vertical rungs to generate a transverse RF field.
Generating high-resolution MRI images is facilitated by employing spatially-homogeneous RF magnetic fields. Spatial variations in the magnetic field can be created by variations in environmental properties as well as system design features. The design of robust, convenient systems yielding spatially-homogeneous RF magnetic fields can be particularly challenging for high RF-frequencies, where common MRI sample losses are relatively high.